[Composer\Repository\RepositorySecurityException]
The contents of http://packagist.org/p/symfony/translation$452e624e2b8bf261
b553b79c0bfc170e5432455ebb33e9fbce585241a5fb7a16.json do not match its sign
ature. This should indicate a man-in-the-middle attack. Try running compose
r again and report this if you think it is a mistake.

This code has worked for me:

composer update --prefer-dist

It timeouts for quite some time, sometimes up to 15 minutes, but then it starts downloading.